1. Field of Invention
This invention relates to apparatus for detecting and amplifying weak impulses of energy absorbed from incident x-ray photons, gamma ray photons, or nuclear charged particles.
2. Discussion of Prior Art
A silicon PIN photodiode or similar photo-active device is often used as a detector for X-ray or gamma-ray photons, electrons (beta particles) or nuclear charged particles. The process of detection may be through direct conversion of collisional energy within the depleted junction region of the diode, or indirectly through intermediate conversion of particle kinetic energy into a flash of light within a scintillating crystal which, in turn, is optically coupled to the photodiode.
In any case, each distinct particle collision imparts a quantity of absorbed energy to the detector producing, in turn, a corresponding quantity of electronic charge. For example: 3.6 electron volts (ev) of absorbed energy are required to produce 1 hole-electron pair in silicon. Thus, a 60 KeV x-ray photon which is completely absorbed produces a pulse containing (60,000/3.6).times.1.6.times.10.sup.-19 =2.667.times.10.sup.-15 coulombs of electronic charge.
Charge-Integrating Preamplifier
In order to measure such small quantities of charge, each charge-pulse is first converted to a voltage signal by means of a charge-integrating preamplifier. An idealized model of a charge-integrating preamplifier--familiar to those skilled in the art--is shown in FIG. 1:
The photodetector is modelled by the parallel combination of a pulsed current-source 11 in parallel with a capacitor 12--typically the junction capacitance plus stray capacitance of the detector diode and its wiring.
The active gain-element of the preamplifier is represented by an idealized, wideband, infinite-gain, low-noise inverting amplifier 2. This preamplifier configuration exploits the well-known "Miller Effect", wherein the infinite-gain, inverting amplifier causes the charge produced by the detector to be deposited in an integrating capacitor 4 (whose capacitance is designated C.sub.int) which is deliberately made much smaller than that of detector capacitance 12. For a given amount of detector charge, Q, the voltage signal amplitude at the output of the amplifier is Q/C.sub.int.
Pulse-Height Spectroscopy
In practice, particle detectors are not used to detect only singular events, but rather a stream of incident particles which, in turn, produces a stream of signal pulses. The amplitude distribution, i.e.--the number of pulses per unit amplitude interval--is termed the "pulse-height spectrum". The intention is to record a pulse-height spectrum which is a true replica of the pulse-amplitude distribution from the detector.
The discharge resistor 5 serves to discharge the integrating capacitor. Without such a discharge element, the voltage on the output of the preamplifier would continue to increase with time due to the sum of leakage currents plus signal currents accumulating as stored charge in the integrating capacitor, causing eventual circuit saturation and amplifier malfunction.
Unfortunately, the discharge resistor may also add substantially to overall system noise. In order to minimize this noise contribution, one must utilize very high values of resistance; 10's, 100's, or even 1000's of megohms are employed for best low-noise performance.
Problems with high-value resistors
However, there are some practical disadvantages to using such high-value resistors. They include:
1. High-value resistors entail long procurement lead times, require "custom" or special-order production and handling, and are relatively high in cost. PA0 2. High-value resistors may still produce "excess noise" above theoretical (thermal noise ) due to "dielectric effects", often requiring selection of individual components for best performance. PA0 3. High-value resistors are not compatible with monolithic integrated circuit fabrication techniques. PA0 4. Active device (FET and PIN diode) leakage currents increase dramatically at elevated temperature. These increased currents, passing through very high value resistors, produce correspondingly large offset voltages, leading to eventual circuit malfunction.
Elimination of the Discharge Resistor
Various methods for performing the discharge function, while eliminating the physical resistor, have been presented in the technical literature. Some solutions entail discharging the integrating capacitor periodically by means of an electronically-actuated switch (Landis, Goulding, et al, "Pulsed Feedback Techniques for Detector Radiation Spectrometers", IEEE Transactions on Nuclear Science Vol. NS-18,1972), or a photo-active switch (Goulding, Walton, and Malone, "An Optoelectronic Coupling Feedback Preamplifier for High-Resolution Nuclear Spectroscopy", Nuclear Instruments and Methods vol. A322 p. 538, 1992). These solutions, while providing excellent, low-noise performance, are rather complex for the application at hand.
Other solutions for effecting a steady--rather that periodic--discharge function, while eliminating high-ohm resistors and still maintaining very low-noise performance, have been put forward by various workers, including Bertuccio, et al., (U.S. Pat. No. 5,347,231), and Fazzi, et al, ("Charge-Sensitive Amplifier Front-end with NJFET and Forward-Biased Reset Diode" IEEE Transactions on Nuclear Science, Vol. 43, No. 6, December 1996).
The former approach utilizes the gate-source junction of a preamplifier's first JFET input stage, operating in a constant forward-biased mode, to effect the discharge function. The latter approach utilizes a forward-biased PN junction diode connected across the input of an amplifier's first JFET.
Both of these utilize a "folded cascode" configuration in order to achieve a high gain-bandwidth product in a relatively simple and compact circuit. As a practical matter--and, as pointed out in the above references--in this type of circuit, where the reset, or discharge function applies only at the amplifier's input terminal, the DC voltage gain is kept low in order to remain reasonably well-centered on a stable, quiescent operating point.
The forward-biased JFET solution of Bertuccio, et al, is effective with small-area diode detectors whose capacitance is on the order of a few picofarads, used in conjunction with small-area JFET devices such as the type NJ26/2N4416 as the first amplifying stage.
However, for larger-area detectors utilizing 1 cm.sup.2 or larger Si PIN diodes such as the Hamamatsu S3590-08, whose capacitance is on the order of 50 picofarads, or more, a larger-area first JFET is required to obtain optimum, low-noise performance. In general, for lowest noise, the input capacitance of the preamplifier's first JFET should match the junction capacitance of the detector diode. This, in turn, requires the use of large-area JFET devices (such as Interfet, Inc. type IF 4501 or equivalent) of quite high transconductance and--as a consequence--impractically large quiescent drain currents when such JFET's are operated in the forward-gate-bias mode.
In the circuit presented by Bertuccio, et al, which is reproduced in FIG. 2, the first JFET is operated in the "triode" or unsaturated region at a drain-source voltage of approximately two volts in order to maintain a reasonable drain current with the gate-source junction forward-biased. However, for the larger JFET's being considered in the present work, the zero-bias saturation drain current is so large (.about.100 milliamperes) and the shunt "on" resistance in the triode operating region so low (a few tens of ohms or less), as to render this approach impractical.
The forward-biased PN junction diode approach of Fazzi, et al, wherein the diode is integrated onto the same monolithic substrate along with the JFET and preamplifier, also utilizes a relatively small-area detector, first JFET, and junction diode. In either case, these solutions are not readily adaptable to a new type of non-inverting charge-integrating preamplifier currently under development.
A Non-Inverting Charge-Integrating Preamplifier
The search for an alternative concept is motivated by the need to develop a discharge method which is compatible with our implementation of a new type of non-inverting charge-integrating preamplifier which, in turn, is the subject of a currently-pending patent application Ser. No. 08/834,089, filed Apr. 14, 1997. The original, idealized concept presented in the aforementioned application is reproduced in FIG. 3:
The detector is once again modelled as a pulsed current source 11b in parallel with a capacitor 12b, but instead of being connected to the inverting input of a high-gain amplifier, the detector is now connected between the output and the input of a unity-gain, non-inverting amplifier 3b. The discharge resistor 5b and integrating capacitor 4b are connected between the input of the amplifier and ground, or circuit common 8.
For an equivalent set of values for the various parameters and components--the charge Q, detector capacitance C.sub.d, integrating capacitance C.sub.int, discharge resistance R.sub.d, etc. the performance of this implementation is exactly equivalent to that of FIG. 1, as suggested by the identical voltage waveforms.
Practical Embodiment of Non-Inverting Preamplifier
A practical embodiment of the, non-inverting charge-integrating preamplifier presented in App'n Ser. No. 08/834,089, filed Apr. 14, 1997, is reproduced in FIG. 4.
As a practical matter, the circuitry comprising the detector and preamplifier assembly is housed in an opaque, metallic enclosure 30 to provide shielding against unwanted ambient light and stray electromagnetic fields. A thin metallic membrane 34 keeps out unwanted light and electromagnetic fields, but allows X-ray photons, gamma-ray photons, or charged particles to impinge on the detector.
Cascode JFET's 61 and 62 comprise the input stage of a unity-gain, non-inverting preamplifier. Diode 59 is back-biased. In this embodiment, the anode of 59 is connected to the gate of JFET 61. Resistor 29 is connected between the cathode of 59 and the detector power supply bus 27. In addition to the bias connection, 29 also provides signal isolation, or decoupling. Resistor 29 has a resistance of 1 megohm or more. The cathode of 59 is also AC coupled to the source (output) terminal of JFET 61 through capacitor 67. For best gain and low-noise performance coupling capacitor 67 must have high capacitance to insure efficient signal-coupling: In this embodiment the capacitance of 67 should be a factor of 100 or more greater than the junction capacitance of 59.
JFET 60 plus resistor 64 function as a high-impedance (current source) load for JFET's 61-62. Capacitor 68, comprising the input capacitance of the amplifier, plus stray wiring capacitance, functions as the integrating capacitor, C.sub.int. The charge-signal, Q'" produces a voltage pulse whose amplitude is (Q'"/C.sub.int). This voltage pulse is applied to the gate of JFET 63, whose source is bypassed to ground by capacitor 66. JFET 63 consequently functions as a high-gain transconductance (voltage-to-current) amplifying stage. The resulting pulsed-current signal I.sub.sig from the drain terminal of 63 is then transmitted to a remote "signal-receiver" and post amplifier by means of a coaxial cable of arbitrary length.
The preamplifier circuit in FIG. 4 utilizes a high-ohm discharge resistor 65. The signal current and leakage current produced in diode 59, plus reverse-gate-leakage current from 61 are all discharged to "ground", or signal common, through resistors 65 and 64.
This circuit has been employed successfully in variety of radiation detector applications. However, its performance can be substantially improved by eliminating the high-ohm discharge resistor.
Objects and Advantages of the Invention
Therefore it is an object of the present invention to implement a steady reset or discharge function in a charge-integrating preamplifier, equivalent to that provided by a high-ohm resistor, but without requiring a physical high-ohm resistor.
A further object of the invention is to achieve low-noise performance in a charge-integrating preamplifier that is substantially equivalent to the low-noise performance employing a high-ohm discharge resistor, but without requiring a physical high-ohm resistor.
Another object of the invention is to implement a discharge element for a charge-integrating preamplifier used in conjunction with relatively large-area (and, in consequence, large-capacitance) diode detectors, and correspondingly large-area first-JFET preamplifier stages.
Another object of the invention is to implement a discharge element which is compatible with a new, non-inverting type of charge-integrating preamplifier which, in turn, is the subject of a currently-pending patent application.
Another object of the invention is to implement a discharge element for a charge-integrating preamplifier which can help mitigate the degradation in circuit performance which occurs at higher ambient temperatures, due to increased detector-diode and preamplifier-input JFET reverse-leakage currents.
Another object of the invention is to implement a discharge element for a charge-integrating preamplifier which is compatible with both discrete-component and monolithic integrated-circuit fabrication techniques.
Other features, advantages, and novel aspects of the invention will become apparent to those skilled in the art from the following specifications and drawings illustrating the underlying concept and examples of practical embodiments.